Trellis coded modulation (TCM) frequently is used in data transmission systems having band-limited channels. By incorporating both coding and modulation, TCM often allows for redundancy because the number of symbols in the resulting constellation is larger than required for the modulation format and for error checking as only certain sequences of symbols are permitted due to the interrelationship between symbols created by convolutional coding.
Referring now to FIG. 1, a typical implementation of TCM in a conventional system is illustrated as described by the International Telecommunications Union (ITU) Recommendation G.922.1 (“the G.992.1 Recommendation”). The G.922.1 Recommendation describes a standard for Asymmetric Digital Subscriber Line (ADSL) systems whereby digital data may be transmitted using analog signaling over relatively long distances. The illustrated conventional TCM encoder 100 often is used to facilitate error control in such transmissions.
In G.992.1 -based systems, the coding of data for transmission generally comprises an outer coding scheme followed by interleaving and then an inner coding scheme, as is well known in the art. TCM typically is implemented as an inner coding technique whereby a sequence of binary words representing the data to be transmitted is coded and mapped to a symbol of a constellation. Further processing of the symbol generally occurs and a representation of this symbol then may be transmitted by analog means to a receiver where it is demapped and decoded to regenerate the original transmitted binary word. To illustrate, a binary word u=(um, . . . , u5, u4, u3, u2, u1), where u1 is the least significant bit (LSB), is processed by providing u1, u2, u3 to a TCM module 102. The TCM module102 typically includes a rate ⅔ or ½ convolution encoder 104 to process u1 and u2 (the next-to-least significant bit) to generate u0 using a convolutional coding technique. The output of the convolution encoder 104 (i.e., u0, u1, u2), along with u3, then is processed by a coset generation module 106 to generate two binary words v=(vm-y, . . . , v1, v0) and w=(wy-l, . . . , w1, w0) from four-dimensional (4D) cosets using mapping by set-partitioning. The binary words v and w then may be mapped by a 4D mapping module 108 to generate a symbol as described by the G.992.1 Recommendation.
Conventional TCM techniques, such as the one described above, often provide an adequate coding gain for a given bit error rate (BER) and spectral efficiency. In certain instances, such as the transmission of data over long distances or in environments having considerable signal impairments, it may be desirable to use a coding technique that provides an improved coding gain compared to TCM. These improved coding techniques, including, for example, convolutional turbo coding, block turbo coding and low density parity check (LDPC) coding, often provide an improved coding gain but at the cost of a lower error floor compared to TCM.